The invention describes a process for the precipitation and separation of cadmium sulphide from the raffinates derived from the purification of phosphoric acid.
In the last few decades, a number of processes have become known for purification of wet-process phosphoric acid by treating it with solvents. Some of these proposals for purification processes have meanwhile been applied in industrial practice. Processes for purification of phosphoric acid have been successfully operated on a large industrial scale for years.
All these processes are characterised by the wet-process phosphoric acid being split up into two production streams: a pure acid stream and a raffinate stream. The latter contains not only a small amount of phosphoric acid but also the sum total of all companion substances from the wet acid and possibly other auxiliary substances which may have found their way into the process in the course of the purification operation. These could be alkaline or ammonium compounds. The cationic companion substances are present in the raffinates as phosphate, sulphate or fluoride salts in concentrated form with a low water content.
The raffinates are valuable by-products whose contents of phosphate and nutritional trace elements make them suitable for use as additives in the fertilizer industry.
The individual substances from the wet acid are enriched in the raffinate in accordance with P.sub.2 O.sub.5 yield in the phosphoric acid pruification process. Filter acids produced from marine phosphate sediments have a small cadmium content in the range from 5 to 50 ppm. This depends on the species of the raw phosphate and is subject to wide fluctuation.
Of late, efforts have been undertaken to minimize the amount of cadmium being introduced into the soil by way of phosphate fertilizers. Although the amounts of cadmium deriving from the raffinates are relatively small, in enriched form they do constitute an important quality feature for the assessment of their suitability as a fertilizer component.
In the last few years a number of processes have become known which aim at reducing the cadmium content in the phosphate fertilizers. These processes can be divided into two groups according how they are applied:
In one group, the cadmium content is reduced in the raw phosphates.
In the other group of processes, the cadmium is separated from the acid.
None of these processes can be applied to the problem of removing the cadmium from the raffinates of the phosphoric acid purification processes, as the physical and chemical characteristics are comparable neither with those of a phosphoric acid nor with those of a raw phosphate. Hitherto, no proposal has been made known for a technical solution to the special problem of removing cadmium from the raffinates of the phosphoric acid purification process.
The raffinates from the phosphoric acid purification processes are highly complex salt mixtures composed of the dissolved and solid companion substances of the wet-process phosphoric acid. Depending on the phosphoric acid purification process, they have a greater or lesser content of free acid. Their degree of saturation lies between the 0.5 and 0.9-basic levels. Their P.sub.2 O.sub.5 content ranges from 25 to 40 percent by weight.
The water content in raffinates ranges between 20 and 30 percent by weight. These are a mixture of a large number of salt pairs and therefore form a strongly oversaturated solution with a tendency toward crystallisation. The raffinates contain a certain amount of solids in the form of microcrystalline bodies embedded in the surrounding, highly viscous solution.
The viscosity of the raffinates usually lies in the range between 100 to 1000 mPas. Raffinates with a higher viscosity are also known. The specific gravity ranges from 1.5 to 1.7 kg/l.
Depending on the origin of the used raw phosphate to produce the wet acid and on the yield of the acid purification process, the cadmium content in the raffinates may range between 20 and 350 ppm.
The removal of cadmium from phosphoric acids by H.sub.2 S precipitation is described in patent literature.
According to DE-OS No. 2422 902, it is possible to precipitate cadmium from more highly concentrated phosphoric acids with a P.sub.2 O.sub.5 content of more than 45 percent by weight under pressure of up to 50 atm and with a residence time of up to 3 hours. The examples show that a residual value of 10% to 90% relative to the cadmium input value remains in the acid. The residual cadmium content in the acid depends on the H.sub.2 S pressure and the exposure time. The sulphide precipitate is separated off by filtration immediately after pressure-gassing in the presence of the high H.sub.2 S content. Following filtration, the H.sub.2 S content is removed from the phosphoric acid by sophisticated technical measures.
The Japanese patent specification No. 75 75.115 also describes an H.sub.2 S pressure-gassing process for removing cadmium from an acid with a P.sub.2 O.sub.5 content of 28%. Here H.sub.2 S is added in the form of a concentrated Na.sub.2 S solution. The example produces an H.sub.2 S concentration in the acid of 0.21 percent by weight, resulting in a pressure of 1.4 atm. The precipitated sulphides are filtered out of the acid containing H.sub.2 S immediately after gassing.
The Japanese patent specification No. 78 75.196 describes a process for the simultaneous reduction of the SO.sub.4 content and of the heavy metal content in a 70% phosphoric acid by treating it with calcium hydroxide in combination with an aqueous Na.sub.2 S solution. This method entails a 25-fold H.sub.2 S surplus calculated on the basis of the heavy metal content. The acid is filtered after a reaction time of 1 hour.
The Swedish patent application No. 7904 135-6 similarly describes a method for cleaning phosphoric acid of heavy metals by precipitation of sulphides. One part of the sulphuric acid is converted into gypsum by calcium hydroxide; the basicity of the free acid is raised by the addition of alkali. The precipitated sulphides are removed from the acid by filtration.
In all known sulphide precipitation processes the precipitated sulphides are removed from the treated phosphoric acid by filtration. It is known that the filtration of sulphide precipitates on an industrial scale poses great difficulties as the sulphide precipitates build up very dense coatings on the filter elements, which in turn results in very low filtration rates.